Heretofore, in producing cellulose acylate films for use in liquid-crystal image display devices, a solution-casting method has been principally carried out, which comprises dissolving cellulose acylate in a chlorine-containing organic solvent such as dichloromethane, casting it on a substrate, and drying it to form a film. Dichloromethane, a type of a chlorine-containing organic solvent has been favorably used as a solvent for cellulose acylate, since it is a good solvent for cellulose acylate and has a low boiling point (about 40° C.), therefore having the advantage of easy vaporization in a film formation step and in a drying step.
Recently, in terms of environment conservation, the leakage of a chlorine-containing organic solvent having a low boiling point is remarkably reduced in a treatment process even in a hermetically closed equipment. For example, the organic solvent was prevented from leaking using a completely closed system or a method of sucking and processing the organic solvent by providing a gas absorbing tower before emitting the organic solvent to outer air although the organic solvent leaks was performed. In addition, the organic solvent is hardly discharged by the decomposition of the chlorine-containing organic solvent using electron beams or combustion due to heating before the discharge, but complete non-discharge need to be further studied.
As such a countermeasure, a method of melting cellulose acylate without an organic solvent and forming a film was suggested (for example, see Patent Document 1). This is to easily melt and form the film by lengthening a carbon chain of an ester group of the cellulose acylate and decreasing a melting point. In more detail, it is possible to melt and form the film by converting cellulose acetate into cellulose propionate or cellulose butylate. However, when the film molten and formed by the above-described method is cut in order to manufacture a polarizing plate, cutting wastage generates and attaches to the film. Accordingly, there is a need for improvement of the cellulose acylate film manufactured by the melt-casting film formation such that the cutting wastage is hard to generate at the time of cutting.
Patent Document 1: JP-A-2000-352620